The main reason, Bai Heng wanted this legacy was because the Blood Sword Emperor had mastered the Law of Blood and the Law of Quietus. He was looking for the way to convert the Saints back to living beings with emotions and free will. He had tried many times to turn them back to their original race but the result was a failure. So, he decided to do, the same thing Morning Star did and started creating a way to convert Saints to a brand new race.